Device for measuring the density and/or specific gravity of a liquid

ABSTRACT

A device which is used to measure the density of a liquid, includes an enclosure ( 1 ) which is impervious to the liquid to be measured. The enclosure ( 1 ) has a rigid deformation-resistant casing housing at least one reference body ( 3 ) which is suspended in the enclosure ( 1 ). The enclosure ( 1 ) can move in relation to the reference body ( 3 ) such as, in the fully submerged state, to occupy a position which is a function of the density of the liquid in which the device is submerged. In the position, the enclosure exerts a pressure or tensile force on a measuring device ( 2 ) the movement of which is limited by the reference body ( 3 ).

The present invention relates to a device for measuring the densityand/or specific gravity of a liquid, of the type comprising an enclosureimpervious to the liquid to be measured.

The measurement of density takes place principally in the monitoring ofcontinuous or discontinuous industrial processes for transformation,such as fermentation, chemical reactions with the release of gas,mixtures, emulsions or the like. Generally speaking, the devices formeasuring density or specific gravity of a liquid (manual densimeterwith derived flow rate, with a vibrating blade, for measuring therelease of gases, etc.) have numerous drawbacks such as difficulty ofreading, sensitive use, high price, high cost of installation, etc.

To overcome such drawbacks, the applicant proposed a device formeasuring the density of a liquid, described in French patentapplication No. FR-A-2.733.318. This device comprises an enclosure ofwhich each of the ends is closed by a flexible membrane which coactswith its internal surface with a transmission member connected to aforce detector at the strain gauge, said enclosure being immersed in avat filled with a liquid whose density is to be measured. This simpledensimeter permits measuring the density of a liquid no matter what itsviscosity. Nevertheless, when it is to measure a loaded liquid, which isparticularly in the case of the agro-food field, this apparatus haszones of retention in which impurities can lodge. This device also has adrawback because of the positioning of the flexible membranes at each ofthe ends of the measuring and closure. Thus, these comprise a largesurface exposed to shock and to projections and can thus be piercedduring operation of manipulation of the apparatus. Finally, thisapparatus has no system for protecting the force detector and thus,under the influence of an overload, the latter can give an erroneousreading of the density of the liquid, and can even deteriorate.

The applicant has moreover proposed, in an International application WO02/01187, a device for measuring the density of a liquid comprising anenclosure impervious to the liquid to be measured, of which each of theends is closed by a wall, these walls coacting together by means of atleast one connecting member. This device is characterized in that itcomprises a securement piece connected to the movable portion of a forcedetector with a strain gauge and secured to one of the walls, the fixedwall of the force detector remaining secured to the enclosure. Such aconstruction requires having an enclosure formed of several elementsthat can move relative to each other. These different elements of theenclosure are connected to each other by means of membranes which areurged in the course of deformation to permit the measurement of thedensity. This design gives rise again to the risk of premature wear ofthe device because of the primary role of the membranes.

Other devices for measuring the specific gravity of a liquid aremoreover known from DE-U-9006275, U.S. Pat. No. 3,589,200, U.S. Pat. No.4,400,978, FR 2.563.339 and JP 59-94036.

An object of the present invention is thus to provide a device formeasuring the density and/or the specific gravity of a liquid whosedesign permits continuous measurement of the product and not of aremoved specimen, this device being free from a deformable enclosurewhose deformation would be connected to the presence of a membrane so ason the one hand to increase the precision of reading of the measurement,and on the other hand to reduce the risks of premature wear.

Another object of the present invention is to provide a measuring devicewhose design permits keeping the sensitive elements of the device, inparticular the measurement members, such as the detector, within asealed enclosure preventing any degradation of these latter.

Another object of the present invention is to provide a measuring deviceof the mentioned type, of extremely simple design, of reducedmaintenance, limited wear, requiring no specialist for its installationand having extreme precision of measurement.

To this end, the invention has for its object a device for measuring thedensity and/or specific gravity of a liquid, of the type comprising anenclosure impervious to the liquid to be measured, characterized in thatthe enclosure is constituted by a rigid indeformable envelope enclosingat least one body, called a reference body, held in suspension in saidenclosure in a manner totally immersed in this latter by means of atleast one suspension member projecting through at least one opening ofthis enclosure closed in a sealed manner, this enclosure being movablerelative to the reference body to occupy a totally immersed condition, aposition relative to said body as a function of the density of theliquid within which the device is immersed, this position being detectedand/or measured by a detection and/or measuring device disposed withinthe enclosure.

Thanks to the design of a measuring device of the density and/orspecific gravity of a liquid in which the enclosure containing themeasurement and/or detection means of the position taken by theenclosure is caused, under the influence of the Archimedes pressureexerted on said enclosure, to move relative to a reference body enclosedwithin the enclosure as a function of the density of the liquid in whichthe device is immersed, there thus results an extremely precisemeasurement of the density. According to a preferred embodiment of theinvention, the enclosure, movable relative to the reference body tooccupy a totally immersed condition, a relative position to the body asa function of the density of the liquid within which the device isimmersed, encloses force measuring means, such as a force detector, thisenclosure exerting, in said position, directly or by means of a piecesecured to move with the enclosure, a compressive or attractive force onthe force measuring means limited in displacement by the reference body,itself insensitive to the forces exerted by said enclosure.

The measurement can thus take place by means of measuring means offorces whose movement is limited by the reference body of which at leastone of the surfaces constitutes, during measurement, a bearing and/orretention surface for the force measuring means, this reference bodybeing insensitive to the forces exerted by said enclosure because of itsdesign.

The invention will be better understood from a reading of the followingdescription of embodiments, with reference to the accompanying drawings,in which:

FIG. 1 is a cross-sectional view of a device according to the inventionoutside of a liquid;

FIG. 2 is a cross-sectional view of the device of FIG. 1 in themeasuring position after variation of the position relative to theenclosure with respect to the reference body;

FIG. 3 is a simplified schematic view of another embodiment of theinvention; and

FIG. 4 is a simplified schematic view of still another embodiment of theinvention.

The measuring device according to the invention permits, as mentionedabove, the measurement of the density and/or the specific gravity of aliquid. This device comprises an enclosure 1 impervious to the liquid tobe measured. In the illustrated embodiment, this enclosure has the formof a body of generally cylindrical appearance, preferably weighted withat least one weight 8, the weight having a function of permitting theenclosure to occupy a balanced position about the axis of thecylindrical body in the immersed condition of the device. This enclosure1 is constituted by a rigid indeformable envelope which is preferablymade of a material determined by the regulations in force in theagrifood industry. The enclosure can thus be made with PVC, stainlesssteel or the like. This enclosure, which thus constitutes a monoblocassembly, encloses at least one body 3, a so-called reference body, heldsuspended in said enclosure 1 in a totally immersed condition of thislatter by means of at least one suspension member 4 projecting throughat least one opening 5 closed in a sealed manner, of said enclosure 1.In the illustrated embodiments, at least one of the suspension members 4is constituted by a tubular element for the passage of the wiresnecessary for the electronics of a force detector 2 also disposed withinthe enclosure 1. This suspension member 4 comprises preferably adjacentits connection with the reference body 3, a rigid portion. In itsportion outside the device, near its securement region, this suspensionmember 4 extends substantially along the axis of the cylindricalenclosure 1 such that the suspension axis coincides with thelongitudinal axis of the cylinder. The suspension member 4 is generallyfixed by its upper portion external to the device, to the wall of a vatenclosing the liquid to be analyzed. The securement should take placesuch that the device will be completely immersed, that it will rest onthe bottom of the vat, and that it will not be hindered in its movementsby touching for example a wall.

The enclosure 1 itself comprises at least one, and preferably two,openings 5 for the passage of a suspension member 4. Each opening 5 isclosed respectively by means of a membrane 6 surrounding said suspensionmember 4. This membrane 6 has, in its uncompressed condition, agenerally conical shape. When inserted in the opening, it delimits atleast one coaxial bellows with the portion of the suspension member 4passing through the opening 5, as shown in FIG. 1. This membrane 6permits relative movement between body 3 and enclosure 1 because of theplay which it establishes at the suspension member 4. The dimensions ofthis membrane are thus a function of the range of movement of theenclosure relative to the reference body 3 and the amplitude of movementpermitted to the enclosure.

In the case in which the enclosure comprises two passage openings 5,these are preferably provided facing each other and can be positioned onthe enclosure coaxially to the vertical suspension axis of the referencebody, as shown in FIG. 4. These openings 5 for passage of a suspensionmember 4 can also be arranged facing each other and positioned along anaxis substantially perpendicular to the vertical suspension axis of thereference body 3, as shown in particular in FIGS. 1 to 3.

This enclosure 1 is thus given a relative movement with respect to thereference body 3 suspended in the enclosure in the course of itsimmersion in the liquid whose density is to be measured, and occupies,in the totally immersed condition, a position relative to the referencebody 3 which is a function of the density of the liquid within which thedevice is immersed. Thus, the position taken by the enclosure 1 is adirect function of the Archimedes pressure on said enclosure. In theimmersed condition, the enclosure 1 thus occupies a position resultingfrom the Archimedes pressure applied against said envelope of theenclosure. Means 2 for measuring and/or detecting the position taken bythe enclosure 1 relative to the reference body 3 in the immersedcondition of the device, are thus provided. These means 2 can have alarge number of forms and generally translate the position taken by theenclosure into a variable electrical signal. In the illustratedexamples, these means are constituted by measuring means of the reactionforce exerted by the enclosure in the immersed position on said means 2.This force can be a compression or a tension exerted by the enclosure onthe means 2 for measuring the force, limited in movement by referencebody 3. These measuring means are for example constituted by a forcedetector 2. Other means for measuring and/or detecting the position ofthe enclosure, such as optical means, could be envisaged in anequivalent manner. They will not be described in more detail hereafter.

Thus, for example, in the case shown in FIG. 2, the enclosure 1 exertson the force detector 2, a compressive force proportional to the densityof the liquid in which it is immersed. The resultant of the forcesapplied to the force detector 2 takes account on the one hand of thetotal weight of the enclosure, constituted by the weight of the envelopeand of the ballast 8 when this latter is present, this ballast beingsecured to the envelope, and of the Archimedes pressure on the otherhand, this Archimedes pressure being a function of the density of theliquid in which the device is immersed. The higher the density, thegreater the Archimedes pressure. There results a greater compression ofthe force detector translated into a higher density. Thus the enclosure1 exerts, in the immersed condition and as a function of the density ofthe liquid within which the device is immersed, a variable compressionor tensile force on the detector, this detector being limited inmovement by the reference body 3, itself insensitive to the forcesexerted by said enclosure. This reference body 3 is thus renderedinsensitive to the forces exerted by said enclosure 1, either thanks toits own weight, which is substantially greater than the forces exertedby said enclosure, or by reason of its mounting through the suspensionmember 4. It must thus be considered that the suspended body 3, once thedevice is positioned, will maintain a constant level and therebyconstitute a reference for the measuring means 2.

Starting from this principle, according to which the detector or anyother equivalent measuring means measures the position taken by theenclosure relative to a reference body, as a function of the density ofthe liquid within which the device is immersed, various embodiments canbe envisaged.

Thus, in a first embodiment of the invention, in the suspended conditionof the reference body 3, the force measuring means are positioned abovethe reference body 3 as shown in particular in FIG. 3. In this case, theforce measuring means, such as a force detector, comprise a fixedportion and a movable portion, the fixed portion being coupled to thereference body 3, the movable portion to the enclosure 1. This detectorcan thus either be subjected to a compression force or a tensile forceas a function of the direction of the force resulting from theArchimedes pressure and of the total weight of the enclosure. In thecase of tensile force, a connection of the detector on the one hand tothe reference body, on the other hand to the enclosure, is necessary. Inthe case of compression, a single connection respectively to theenclosure or to the reference body is necessary provided that theresulting force will be such that the enclosure applies itself in allcases to the detector. In the embodiment shown in FIG. 4, the enclosureexerts, by means of a piece 9, such as an internal partition secured inmovement to the enclosure, a compressive force on the force detector 2.

In another embodiment of the invention according to FIGS. 1 and 2, inthe suspended condition of the reference body, the force measuring meansare positioned below the reference body 3. In this embodiment, the forcemeasuring means can be secured to an internal wall of the enclosure orelse the force measuring means can be secured to the reference body.These force measuring means can also be secured to an internal wall ofthe enclosure and secured to the reference body, the fixed portion ofthe force measuring means being secured to the reference body, themovable portion to the internal wall of the enclosure. FIG. 2 showsrelative to FIG. 1 the position taken by the enclosure after immersionin the liquid.

Independently of their position, these force measuring means can beconstituted by a strain gauge, a pressure detector, a force transmitteror a resistance for detection of forces as is well known to thoseskilled in this art. In the illustrated examples, the detector is adeformable body, deformed as a function of the position taken by theenclosure 1 relative to the reference body 3, this position being adirect function of the density of the liquid to be analyzed. The signalstransmitted by the detector 2 translate this deformation of the detectorbody. To permit analysis of the results obtained from the informationsupplied by said detector, this detector or any other equivalentmeasuring means is connected to means for processing and analyzing inreal time or differentially and preferably continuously the signalsproduced by the detector. These processing and analyzing means, notshown, can be constituted by a computer incorporating software for theacquisition and processing of data connected to said detector. Such adevice can also comprise a display to express the obtained results.Generally speaking, these means convert a tension into a digital value.This device also comprises means for generating the regular supply forthe detector. This device can also comprise means for measuring thetemperature, this temperature being displayed simultaneously or in analternating manner with the information relative to the density and/orspecific gravity. Thus, the processing of the data permits displaying,from information relative to the density and information relative to thetemperature, the specific gravity of the liquid in which the device isimmersed. As a detector, there can also be used a piezoelectric detectorcoupled to a load converter. This configuration has the advantage ofhaving no aging of the detector with time and provides excellentsensitivity of the detector to rapid variations and linearstandardization. The use of a force detection resistance as a detector,associated with a resistance-tension converter, permits making thedevice particularly affordable. All these types of detectors have theiradvantages and their drawbacks well known to those skilled in the art.

The device for measuring density and/or specific gravity described abovewill preferably be used in vats having a depth less than 10 meters formeasurements of the density of the liquid comprised within the range of950 g/l-1300 g/l for temperatures varying between 15° C. and 40° C.

It is to be noted that the presence of ballast 8 of the enclosure 1permits giving a measurement within a predetermined range and permitsobtaining a reaction of the detector within a predetermined range bylimiting the amplitude of movement of the enclosure.

1. Device for measuring the density and/or specific gravity of a liquidof the type comprising an enclosure (1) impervious to the liquid to bemeasured, characterized in that the enclosure (1) is constituted by arigid indeformable envelope enclosing at least one reference body (3),said reference body being held in suspension in said enclosure (1) in atotally immersed condition of said enclosure by means of at least onesuspension member (4) projecting through said at least one opening (5)closed in a sealed manner, of said enclosure (1), this enclosure (1)being movable relative to the reference body (3) to occupy, in thetotally immersed condition of the enclosure, a position relative to thebody (3) as a function of the density of the liquid within which thedevice is immersed, this position being detected and/or measured bydetection and/or measurement means (2) disposed within the enclosure(1).
 2. Measuring device according to claim 1, characterized in that theenclosure (1), movable relative to the reference body (3) to occupy, inthe totally immersed position, a position relative to the body (3) as afunction of the density of the liquid within which the device isimmersed, encloses means (2) for measuring force, such as a forcedetector, this enclosure exerting, in said position, directly or bymeans of a piece (9) secured to move with the enclosure, a compressiveor tensile force on said means (2) for measuring forces limited indispersement by the reference body (3), itself insensitive to the forcesexerted by said enclosure (1).
 3. Measuring device according to claim 2,characterized in that, in the suspended condition of the reference body(3), the force measuring means (2) are positioned above the referencebody (3).
 4. Measuring device according to claim 2, characterized inthat, in the suspended condition of the reference body (3) the forcemeasuring means (2) are positioned below the reference body (3). 5.Measuring device according to claim 2, characterized in that the forcemeasuring means are constituted by a force detector (2) comprising afixed portion and a movable portion, the fixed portion being coupled tothe reference body (3), the movable portion being coupled to theenclosure (1).
 6. Measuring device according to claim 2, characterizedin that the force measuring means (2) is secured to an internal wall ofthe enclosure (1).
 7. Measuring device according to claim 2,characterized in that the force measuring means (2) are secured to thereference body (3).
 8. Measuring device according to claim 1,characterized in that the enclosure (1) comprises at least two openings(5) for the passage of a suspension member, each opening (5) beingclosed respectively by means of a membrane (6) surrounding saidsuspension member (7).
 9. Measuring device according to claim 8,characterized in that the openings (5) for passage of a suspensionmember are arranged facing each other and are positioned on theenclosure in a manner coaxial with the vertical axis of suspension ofthe reference body (3).
 10. Measuring device according to claim 8,characterized in that the openings (5) for passage of a suspensionmember are arranged facing each other and are positioned on an axissubstantially perpendicular to the vertical axis of suspension of thereference body (3).
 11. Measuring device according to claim 1,characterized in that at least one of the suspension members (4) isconstituted by a tubular element for the passage of wires necessary forthe electronics of the measuring means (2).
 12. Measuring deviceaccording to claim 1, characterized in that the enclosure (1) has agenerally cylindrical shape, preferably ballasted.
 13. Measuring deviceaccording to claim 1, characterized in that the measuring and/ordetecting means (2) are connected to means for processing and analyzingin real time or differentiated and preferably continuously, the signalsproduced by said measuring means (2).
 14. Measuring device according toclaim 2, characterized in that the force measuring means (2) areconstituted by a deformable body deformed as a function of the positiontaken by the enclosure (1) with respect to the reference body (3), thisposition being a direct function of the density of the liquid to beanalyzed.